To start building Qt 5 apps you will need to export QT_SELECT=qt5, install the package qt5-defaults or read qtchooser's man page. Note that exporting QT_SELECT has precedence over installing qt5-defaults.

What's not there

Architectures

AMD64 is already there because it's the arch used by maintainers to build the packages. i386 should be following as soon as buildds catch up. Most surely ARM-based archs will be there at some point too.

Other archs will need some love. Not strange, the Qt project supports amd64, i386 and ARM, but we Debian have normally prepared patches to make it build in other archs. And yes, we try to push them upstream for everyone's benefit. So, if you are missing it in your arch, take a look. You may be the one who enables Qt 5 in it :-)

GLES2 and Wayland

We don't have GLES2 or Wayland support yet. Building it will most probably break the desktop for people using proprietary video drivers (or at least I was told so). I'll surely provide non-official packages with GLES2/Wayland enabled to allow people testing it, but not soon.

This also means that we are not currently able to split X11 and framebuffer support. But we have time to work on it :-)

Non DFSGs compliant files

If you get the original source code tarball from Debian you will notice that it has dfsg in it's name. That means that we had to remove some non DFSG compliant stuff from the original tarball, namely: